Some points remember to HttpServlet is abstract class :
1)Its default implementations of the main service methods do nothing.
2)It must be overridden.
3)It implements the Servlet interface conveniently.
4)It means that developers do not need to implement all service methods.
5)If servlet is required to handle request used doGet(),there is no need to write a doPost().
For read more information :http://www.roseindia.net/servlets/introductiontojavaservlet.shtml